A refractometer is a device for measuring the refractive index of a liquid substance. Refractometers are commonly used in industry where it is advantageous to know the specific relationship between two chemical species in a binary mixture.
When light passes from one medium (n1) into another (n2) with a refractive index different than the first, the relationship between the angle of incidence and the angle of refraction can be represented by Snell's Law: EQU Refractive index(n1).multidot.Sin(Angle of Incidence)=Refractive index(n2).multidot.Sin(Angle of refraction)
When the angle of incidence exceeds a certain angle, all light ceases to be refracted and is instead total internally reflected at the boundary between the two mediums. This angle of incidence is the "critical angle" and is a well documented scientific principle.
Critical angle is defined as: EQU .phi..sub.C =sin.sup.-1 (n2/n1)
Where n1 is the refractive index of the first, light incident, medium and n2 is the refractive index of the second medium.
Known critical angle type hand-held refractometers typically incorporate a series of optical elements arranged to measure the refractive index of a solution under test. These optical elements may consist of a primary critical-angle prism, a transparent sample cover, an objective lens, a reticle, and a focusing means. All the elements are combined together and properly positioned and aligned in a rigid body or housing. Numerous other configurations of the internal components are possible including the addition of temperature compensating means, and various arrangements of optical components.
In operation, the liquid measurand is sandwiched between the measuring surface of a prism and an externally mounted transparent cover. Light incident upon the measuring surface of the prism passes through the exit surface, at an angle relative to the measuring surface. An objective lens focuses this light on an image plane or reticle. The reticle is usually a small piece of glass or other transparent material with an analog scale image etched vertically across its face. Tiny horizontal divisions divide the vertical scale and are representative of some specific unit of measure related to the refractive index of the solution.
Light focused on the image plane creates a shadowline or demarcation, at the critical angle, which separates the field of the reticle into light and dark regions. Very small changes in the refractive index of the liquid under test will create corresponding small vertical movement in the shadowline. It is not necessary to describe in any greater detail the path of the light energy through the optical elements or the formation of the image on the image plane, since it is elucidated in numerous patents and textbooks and is well known to those skilled in the art of refractometry.
A few refractometers with illuminating means have previously been disclosed in U.S. Pat. Nos. 2,601,128 and 4,650,323 and most notably is U.S. Pat. No. 4,890,916. These prior art refractometers have included illuminating means attached to a refractometer body or housing in a hinged sample cover superior to the measuring surface of a prism.
One disadvantage of the prior art refractometers is that the illuminating means is positioned in a hinged sample-cover suspended above the measuring surface. When the power source and electronics are housed in the sample cover, the cover becomes bulky and these elements can in fact impede ambient light passing through the cover, as well as create secondary shadows and/or reflections.
Another disadvantage to the prior art is that the sample cover is prone to damage since it is mounted externally on the refractometer body. Most importantly, the prior art provides for general illumination perpendicular to the plane of the measuring surface instead of at a preferred grazing angle of incidence.
A further disadvantage of prior art is that pressure induced by pressing on the sample cover to actuate the light source can squeeze the liquid sample from between the sample cover and the measuring surface or cause enough mechanical pressure on the prism to actually move the prism with respect to the other optical components causing an erroneous reading. Another disadvantage of this invention is that the amount of space available for mounting a light source and battery limits the size and life of the battery.
Various advantages of the present invention distinguish it from prior art. Because the illumination of the present invention is supplied at either a grazing incidence to the measuring surface or from below the measuring surface, light is incident relative to the prism in the direction necessary to best enhance the measurement of refractive index. Further, since the illumination means is provided in the body or housing of the refractometer it is sturdier and not prone to breakage or damage as in an externally mounted illuminating means.